I think you're misunderstanding them.
If you think the joke is about her appearance then yes the exact appearance matters (which would be valid, if this would be about unixsocks). The characters are what's important to me here instead.
Lain is a computer enthusiast, who is kind of a shut in (at first). The above line is out of character for her but not Misato. If you don't know these people then you wouldn't know this, even if you have perfect vision.
Alt text should describe what you're communicating, not necessarily every detail.